Proper repair of tissue after injury depends on correct wound healing, a multistage process that involves different cell types for each step. Singer and Clark, N Engl J Med, 341(10): 738-46 (1999); Hosgood, Vet Clin North Am Small Anim Pract, 36(4): 667-85 (2006). Wound healing is characterized by three overlapping phases: inflammation, tissue formation, and tissue remodeling. One key event during tissue formation involves fibroblasts invading the wound space composed largely of fibrin and fibronectin, termed the provisional matrix. Clark, Ann N Y Acad Sci, 936: 355-67 (2001); Broughton et al, Plast Reconstr Surg, 117(7 Suppl): 12S-34S (2006); Corbett and Schwarzbauer, Trends Cardiovasc Med, 8(8): 357-62 (1998). Once the fibroblasts populate the wound site, they produce provisional secondary extracellular matrix, consisting primarily of fibronectin, tenascin, and hyaluronan, which begins the formation of granulation tissue. Ghosh et al., Tissue Eng, 12(3): 601-13 (2006); Mimura et al., J Invest Dermatol, 122(6): 1390-8 (2004).
This newly synthesized secondary matrix directs repair by supporting and regulating functions of cells recruited to the wound site including cell proliferation, migration, and angiogenesis. Eventually, collagen is deposited and the new matrix is further remodeled and contracted. Given the critical role that fibroblasts play in creating the matrix, it is important to understand what drives fibronectin secretion and assembly of fibrils.
The provisional matrix initially utilized by fibroblasts can have a positive or negative effect on the ability of fibroblasts to function. While growth factors and the matrix proteins guide the migration of fibroblasts into the provisional matrix, the assembly of the new fibronectin-rich secondary matrix that fibroblasts secrete and assemble, which begins after migration has stopped, can be driven by many cues. Riedel, K., et al., Int J Mol Med, 17(2): 183-93 (2006); Rumalla, and Borah, Plast Reconstr Surg, 108(3): 719-33 (2001); Roy, P., et al., Cell Motif Cytoskeleton, 43(1): 23-34 (1999). Fibrillar fibronectin assembly depends on fibronectin-integrin interactions to create a new matrix in a step-wise fashion. Mao and Schwarzbauer, Matrix Biol, 24(6): 389-99 (2005). First, the α5β1 integrin binds to soluble fibronectin at the cell-binding region encompassing the synergy sequence (PHSRN) located in the 9th type III repeat and the adjacent Arg-Gly-Asp (RGD) cell-binding sequence in the 10th type III repeat. Aota et al, J Biol Chem, 269(40): 24756-61 (1994); Danen, et al., J Biol Chem, 270(37): 21612-8 (1995); Main, et al., Cell, 71(4): 671-8 (1992); Leahy et al, Cell, 84(1): 155-64 (1996). After the cell binds to fibronectin, molecular events leading to the reorganization of the actin cytoskeleton and activation of signaling complexes take place, leading to the elongation and stretching of fibronectin from its compact form. Finally, fibronectin-fibronectin interactions lead to assembly formation. The cytoskeleton plays a critical role during matrix assembly, as the cytoskeletal state mediates the activation of signaling events that promote the assembly of fibronectin fibrils. In the past, biochemical approaches have been used to modulate the cytoskeleton to promote assembly of fibronectin fibrils.
More recently, biophysical approaches have been used to investigate how interfaces could modulate matrix assembly. Fibroblasts have been documented to respond to both spatial and mechanical input of integrins, affecting cell shape and potentially gene expression and cell functions. Three dimensional fibronectin scaffolds have been used to examine the effect of dimensionality on matrix assembly. Chiquet, M., et al., Matrix Biol, 22(1): 73-80 (2003); Mao and Schwarzbauer, J Cell Sci, 118 (Pt 19): 4427-36 (2005); Cukierman, E., et al., Science, 294(5547): 1708-12 (2001).
In these studies, cell associated matrix assembly was greater in three-dimensional substrates compared to two-dimensional substrates. The degree of rigidity of the substrate, which influences the ability of the cell to contract the substrate, has been shown to play a role in matrix assembly, where rigid substrates promote better cell attachment, and allow the cells to elongate and contract, leading to greater quantities of matrix assembly compared to compliant substrates.
Given the complexity and dynamic nature of cell interactions with matrix ligands, a need exists for a better understanding of matrix-cell interactions which would lead to novel methods and compositions for tissue engineering and wound healing.